1. Field of the Invention
The present invention generally relates to a method for converting and/or detecting data and, more particularly, is directed to a method for converting and/or detecting data suitable for recording data at a high density.
2. Description of the Prior Art
Conventionally, in a digital signal magnetic recording and/or reproducing apparatus, binary values "1" and "0" of a digital signal are basically associated with the presence or absence of polarities of magnetization or inversion of magnetization. Further, in case of modulating (writing) a digital signal, the binary values are suitably associated with the magnetization in one bit unit by coding a recording signal in accordance with various modulation methods such as the non-return to zero (NRZ), frequency modulation (FM), modified frequency modulation (MFM) and group coded recording (GCR) (eight-to-ten conversion) etc., depending on physical characteristics of a recording medium and a transmission band of a system etc.
In case of demodulating (reading) the signal, the differential detection or integration detection is selectively used depending on the DC components of the modulated codes to thereby demodulate the data by detecting the signal in one bit unit.
In the above-described conventional digital signal recording and/or reproducing apparatus, it is premised that there is no inter-symbol interference and so a level of the reproduced signal is required to be sufficiently high in a high frequency band. Namely, a maximum repetition frequency and recording density of information are decided depending on a signal-to-noise ratio (S/N) of a high frequency band reproduced signal corresponding to a minimum magnetization inverting interval (Tmin) of the various modulation codes.
Thus, as shown in FIG. 1, a present maximum repetition frequency f max is set at a position where a high S/N ratio is obtained in a decreasing area of a reproducing level of a reproducing- level vs. frequency characteristics. In this decreasing area, a level of the reproduced signal decreases at a gradient of 12 dB/oct, for example, due to various losses in the recording and reproducing modes.
Further, an ideal transmission characteristics (frequency spectrum) as shown in FIG. 2A has been required and so such equivalent characteristics with a level decreasing area of a sine wave configuration satisfying the first criterion of Nyquist as shown in FIG. 2B has been utilized.
Now, in FIGS. 2A and 2B, a frequency fo corresponds to the minimum magnetization inverting inverval (T min).
The first criterion of Nyquist is such a condition that, when a signal wave is sampled at every constant period in a receiver side, sampled values other than the center point become 0.
In the eight-to-ten conversion, for example, utilized in a present digital audio tape recorder (DAT) etc., a relative speed between an application type metal tape (MT) and a magnetic head is slightly larger than 3 m/sec. and a maximum repetition frequency f max is set to be 4.7 MHz and further a gap length and a recording waveform are set to be 0.25 .mu.m and 0.67 .mu.m, respectively. In this case, a track pitch and a linear recording density will be about 10 to 15 .mu.m and 60 to 80 kbpi at their limits, respectively.
Now, one way of increasing the recording density is to increase the maximum repetition frequency f max. However, if the maximum repetition frequency is increased to be twice as large as f max in order to double the recording density twice, for example, a level of the reproduced signal decreases at the frequency 2.multidot.f max when compared with that at f max as shown in FIG. 1 to degrade the S/N ratio remarkably, so that detection of the data becomes impossible.
Thus, the present magnetic recording and/or reproducing apparatus uses the recording medium and the signal conversion unit at their limits, so that it is quite difficult to decrease various losses at the recording and reproducing modes so as to improve a level of the reproduced signal at a high frequency band remarkably.
On the other hand, there occurs such a problem of inter-symbol interference in the reproduced waveform if the recording density is increased.
Namely, when there is one magnetization inversion isolatedly on the recording medium, a reproducing signal thereof will be a pulsative voltage waveform (isolated pulse) as shown in FIG. 3. This isolated pulse, that is, a waveform of an impulse response can be approximated to a Lorense type waveform represented by a following equation (1), for example, and the degree of spread of the waveform on a time axis (pulse width}is determined in accordance with the total transmission characteristics of the recording and reproducing system and the magnetic recording medium to be used. The pulse width is represented normally by a half width Wh at a level of 50 % of a peak level or a width Wb at a base level which is substantially 0 % of the peak level. EQU f(t)=1/{1+(t/to).sup.2} ( 1)
If there are a plurality of magnetization inversions continuously at a constant space, as long as the recording density is low, there is no interference between adjacent pulses in a reproduction mode, so that the reproduction signal will be merely a sequence of alternations of inversed isolated pulses as described above.
If the recording density is increased to such a degree that an interval between adjacent pulses becomes a half of the pulse width Wb at the base level, end portions of the adjacent pulses overlap to each other as shown in FIG. 4 and so the waveform of the reproduced signal will be quite different from that of the isolated pulse.
However, as clear from FIG. 4, information as to peak values of the respective pulses in this state can be maintained without being distorted and so there occurs no inter-symbol interference regardless of the existence of interferences between the pulses.
If the recording density is increased much more than that of FIG. 4, a peak value of the reproduced signal decreases and a non-linear inter-symbol interference (peak shift) where an interval between the peak positions becomes larger will be generated.
Further, if the recording density is more increased to such a degree that an interval of the adjacent pulses becomes one fourth of the pulse width Wb at the base level as shown in FIG. 5, for example, the reproduced waveform of the pulses will be similar to a sine waveform and a peak value is remarkably decreased, and further there occurs the inter-symbol interference which degrades information of peak values of the respective pulses.
By the way, the partial response system (PR system) has been known as a method of using the inter-symbol interference.
This partial response method limits the frequency spectrum within the Nyquist band width as shown in FIG. 6, for example, by suitably constructing codes to thereby require no high-frequency component advantageously.
The transmission characteristics of FIG. 6 corresponds to a class 4 of the partial response (modified duo binary) and can be represented by a following equation (2). EQU Pr(1, 0, -1)=sin (2.pi.f/fo) (2)
However, since the above-described various modification codes are prepared without taking into consideration the inter-symbol interference, the above-described advantages of the partial response method could not been satisfactorily obtained even if the method is applied to the data conversion and detecting method.
Further, a maximum magnetization transition interval (Tmax) becomes infinite in some of the modulation codes, so that such functions required in the system as the overwriting and the clock reproduction could not have been realized disadvantageously regardless of the application of the partial response method.